Dry eye, also know as keratoconjunctivitis sicca, is a multifactorial disease of the tears and ocular surface that results in symptoms of discomfort, visual disturbance, and tear film instability with potential damage to the ocular surface. It is accompanied by increased osmolarity of the tear film and inflammation of the ocular surface (The Ocular Surface, “The Definition and Classification of Dry Eye Disease: Report of the Definition and Classification Subcommittee of the International Dry Eye Workshop (2007),” 5(2): 75-92 (2007)). Dry eye is recognized as a disturbance of the lacrimal functional unit, an integrated system comprising the lacrimal glands, ocular surface (cornea, conjunctiva and meibomian glands) and lids, and the sensory and motor nerves that connect them. The lacrimal functional unit controls the major components of the tear film in a regulated fashion and responds to environmental, endocrinological, and cortical influences. The unit's function is to preserve the integrity of the tear film, the transparency of the cornea, and the quality of the image projected onto the retina. Disease or damage to any component of the lacrimal functional unit (the afferent sensory nerves, the efferent autonomic and motor nerves and the tear-secreting glands) can destabilize the tear film and lead to ocular surface disease that expresses itself as dry eye.
The major classes of dry eye are aqueous tear-deficient dry eye (ADDE) and evaporative dry eye (EDE). ADDE is due to failure of lacrimal tear secretion and this class can be further subdivided to Sjogren syndrome dry eye (the lacrimal and salivary glands are targeted by an autoimmune process, e.g., rheumatoid arthritis) and non-Sjogren's syndrome dry eye (lacrimal dysfunction, but the systemic autoimmune features of Sjogren's syndrome are excluded, e.g., age-related dry eye). EDE is due to excessive water loss from the exposed ocular surface in the presence of normal lacrimal secretory function. Its causes can be intrinsic (due to intrinsic disease affecting lid structures or dynamics, e.g., meibomian gland dysfunction) or extrinsic (where ocular surface disease occurs due to some extrinsic exposure, e.g., vitamin A deficiency) (See The Ocular Surface, “The Definition and Classification of Dry Eye Disease: Report of the Definition and Classification Subcommittee of the International Dry Eye Workshop (2007),” 5(2): 75-92 (2007)).
Dry Eye is one of the most common ocular problems with an estimated prevalence of 4.91 million people in the United States affecting around 3.23 million women and 1.68 million men over the age of fifty (The Ocular Surface, “The Epidemiology of Dry Eye Disease,” 5(2): 93-107 (2007)). Current therapies for dry eye are palliative with a focus on the replacement of tears to reduce symptoms. Over-the-counter artificial tear formulations are available. In addition, a non-pharmacological approach for improving aqueous tear film content is punctual tamponade occlusion. However, punctual tamponade occlusion carries the risk of reduced tear production, clearance and ocular surface sensation. While these palliative therapies have benefits over the short term, they have limited utility in long-term control therapy for dry eye. RESTASIS® (cyclosporine A) is the first prescription product for dry eye therapy. RESTASIS® increases tear production in patients whose tear production is suppressed as a result of ocular inflammation associated with dry eye disease. However, there is a need for therapies that have a broader application than anti-inflammatory medication.
Several clinical studies have found that topical NGF improves the corneal sensitivity in dry eye and increases the number of conjunctival goblet cell density in a study of dogs with surgically induced dry eye (Bonini, S., et al., “Topical Treatment with Nerve Growth Factor for Neurotrophic Keratitis,” Ophthalmology, 107: 1347-1352 (2000)). However, due to the fact that NGF stimulates neurite sprouting by neural cells, one of the side effects of administration of topical NGF is ocular pain (Bonini, S., et al., “Topical Treatment with Nerve Growth Factor for Neurotrophic Keratitis,” Ophthalmology, 107: 1347-1352 (2000)). In addition, NGF has poor pharmacokinetics and bioavailability and the costs for manufacturing are high. A need exists in the art for alternative methods of treating dry eye.